Conventional PTC heating wires are arranged as shown in FIGS. 1 and 2. The wire of FIG. 1 has cores 1, 1' and metallic foil electrodes 2,2' spirally wound, rspectively, about the cores, which are entirely covered with a PTC resistor 3 and an insulative sheath 4 in this order. The wire of FIG. 2 includes a core 1, which is covered, as shown, with an electrode 2, a PTC resistor 3, an electrode 2' and an insulative sheath 4 in this order. When these PTC heating wires are energized by application of a voltage between the electrodes 2 and 2', the electrodes 2,2' as well as the PTC resistor 3 generate heat. The amount of heat generated from the electrodes 2,2' depends chiefly on the electrode resistance and the electric current, and the heat generated in the electrode is greater at a portion which is nearer to the voltage-applied point. This is considered for the reason that the electric current passing through the electrodes 2,2' is greater at a portion nearer to the voltage-applied point because of the leakage current from the electrodes 2,2' to the PTC resistor. This leads to the fact that when the resistance of the electrode per unit length is high, the leakage current to the PTC resistor 3 becomes great with a wide distribution of the heat in the electrode. FIG. 3 is a schematic view of wire connections which enable the drop of voltage by the electrode resistance to be minimized and also the non-uniformity of generated heat along the heating wire to be minimized. As shown in the figure, a voltage is applied between one end of the electrode 2 and the other end of the other electrode 2'. In these wire connections, when the ratio of the electrode resistance to the PTC resistance is high, the distribution of a generated heat density becomes great. The electric circuit of the PTC heating wire using the wire connections will be shown in FIG. 4. The PTC heating wire involves a "ladder-type circuit" of the resistances of the electrodes 2, 2' and the resistance of the PTC resistor 3. Assuming that the heating wire is cut to unit length, a resistance of unit length of one electrode is represented by R.sub.E and a volume specific resistance under stable conditions of the PTC resistor per unit length is represented by R.sub.PTC. L means a unit conduction path length of the PTC heating wire. In the model circuit of FIG. 4, the density distribution becomes greater at a higher value of R.sub.E. If the distribution is too wide, such PTC heating wire cannot stand practical use.
Moreover, if the electrode resistance is high, the heat generated in the electrode becomes great, presenting the safety problem. In particular, when a continuous PTC heating wire is applied as electric articles of high electric capacity, the electrodes 2, 2' reach high temperatures under abnormal, heat-insulated conditions because of the absence of self-temperature control function and thus the heating wire cannot be safe.
In order to solve the problem, it is necessary to reduce the electrode resistance. However, if the electrode resistance is reduced limitlessly, other two problems may take place depending on the conditions for use. One of the problems is that for better electric conductivity, the electrodes 2,2' must have a larger size with a difficulty for mounting. The larger size of the electrodes 2,2' involves not only the difficulty for their mounting, but also the very high possibility of damaging the PTC resistor 3 on bending and breaking the electrodes 2,2' per se.
Another problem may be left even after removal of the limitation on the mounting as described below.
If the electrode resistance is made small, the drop of voltage caused by the electrodes 2,2' becomes small with a small distribution of generated heat. This makes a small amount of heat generated in the electrodes, so that most of heat generated in the PTC heating wire is attributed to the heat from the PTC resistor 3. The electric current passing through the PTC heating wire depends largely on the resistance of the PTC resistor 3 and thus the ratio of a rush current at the time of commencement of energization and a current at the time of stable energization (hereinafter referred to simply as rush current ratio) is dependent fully on the PTC characteristic. If the rush current at the time of commencement of energization is permitted to pass through the PTC heating wire of a continuous form in amounts two or more times the current under stable conditions, abnormality is apt to occur locally, leading to a serious safety problem of breakage or burning of the PTC heating wire. For instance, when the PTC heating wire is applied to ordinary domestic heating appliances and the PTC characteristic of the PTC resistor 3 is such that the temperature coefficient at 70.degree. C. is about 3 times higher than at 20.degree. C. with respect to resistance as particularly shown in FIG. 5, the rush current at 20.degree. C. will exceed 2000 W provided that the electric power under stable conditions is 700 W. In addition, the distribution of heat generation is very wide. To avoid this, it may occur to one that a PTC resistor, which has a smaller temperature coefficient than the temperature coefficient of the PTC resistor shown in FIG. 5, is used. However, this is disadvantageous in that the self-temperature control function of the PTC heating wire is weakened, thus laking stabilities against variations of voltage, room temperature and load. In this sense, the use of such PTC resistor is not appropriate for fabrication of a heating wire utilizing the PTC characteristic.
As will be appreciated from the above, when the electrode resistance of the PTC heating wire is too high, there are involved several problems that the distribution of heat generation is so great that the heating wire cannot stand practical use and that the heating wire becomes hot under abnormal heat-insulated conditions, so that the safe service of the wire is not ensured. On the other hand, when the electrode resistance is too small, the afore-described mounting and safety problems are produced.